This invention relates to antennas in general and particularly to a helical antenna for use with a portable radio device. The use of helical antennas is well known in devices operating in the VHF and lower portion of the UHF range. Such antennas are physically shorter than both conventional quarter wave whip type antennas and dipole antennas.
Conventionally, helical antennas, for the aforementioned frequency ranges, are constructed by winding a helical coil, attaching the coil to the antenna connector, encasing the coil in a plastic sleeve, trimming the coil to desired resonance and placing a cap over the upper trimmed end of the antenna. Such antennas must be trimmed for frequency resonance after construction, due to the fact that various parameters such as the pitch of the helix can be changed during construction. It is, therefore, not possible to precut the antenna to the desired resonant frequency utilizing normal construction techniques.
A further problem exists in attempting to utilize such antennas at higher UHF frequencies such as in the 800 and 900 megahertz ranges. At such frequencies, the antennas become extremely sensitive to slight variations in the dimensions of the antenna. Such deformations can occur from flexing or impact to the antenna and can result in permanent deformation thereby placing the resonance of the antenna out of the desired range.